1. Field of the Invention
The present invention relates to a method of improving physical properties of titanium alloys and, more specifically, a method of increasing thermal conductivity and reducing electrical resistivity of articles made of titanium-based compositions.
2. Description of the Background Art
Titanium alloys offer attractive physical and mechanical property combinations that provide significant weight savings in various industries such as aerospace and space. Thermal conductivity of titanium alloys, however, is low compared to other structural metals such as steel and aluminum. Low thermal conductivity of titanium alloys affects heating rates and obtainable cooling rates after processing and heat treatments. Another drawback of titanium alloys is their high electrical resistivity compared to steel and aluminum. High electrical resistivity limits the use of titanium alloys as electrical conductors. There is a need, therefore, for a new and improved method of increasing thermal conductivity and reducing electrical resistivity of conventional titanium alloys such as Ti-6Al-4V without debits in mechanical properties, specifically tensile elongation and fatigue. The method of the present invention meets this need.
In accordance with the new and improved method of the present invention, titanium boride (TiB) precipitates are incorporated into a titanium alloy and the alloy is then subjected to controlled deformation to orient the TiB precipitates in the direction of interest to achieve improvements in thermal and electrical properties. The controlled deformation of the alloy to orient the TiB precipitates is accomplished by hot metalworking.
The boron is introduced into the titanium alloy composition to produce TiB precipitates by any suitable method, such as casting, cast-and-wrought processing, powder metallurgy techniques such as gas atomization and blended elemental approach. Hot metalworking operations such as forging, rolling and extrusion can be used to accomplish alignment of the TiB precipitates along the direction of metal flow.
As an illustrative example, the method of the present invention may be used to increase thermal conductivity and reduce electrical resistivity of multi-component titanium alloys such as Ti-6Al-4V (Ti-64) and Ti-6Al-2Sn-4Zr-2Mo(Ti-6242).
a is a microstructure of Ti-6Al-4V-1B showing a cross-section of as-atomized pre-alloyed powder particle;
b is a microstructure of Ti-6Al-4V-1B after powder consolidation via hot isostatic pressing;
a is a microstructure of an extrusion article made out of pre-alloyed powder of Ti-6Al-4V-1B revealing TiB precipitates (dark phase) aligned along the extrusion axis.
b is a transverse micrograph of
Methods of increasing thermal conductivity and reducing electrical resistivity of multi-component titanium alloys such as Ti-6Al-4V (Ti-64) and Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) are described hereinafter. These methods encompass two important elements:
Introduction of boron into the titanium alloy composition to produce TiB precipitates can be accomplished by several different methods, such as casting, cast-and-wrought processing, powder metallurgy techniques such as gas atomization and blended elemental approach. The boron may be added to the titanium alloy in the liquid state, wherein the boron is completely dissolved in the liquid titanium alloy. The boron may be added to the titanium alloy through intermixing of solid powders, as by powder metallurgy. Regardless of the process used to add the boron to the titanium alloy, the boron may be added as elemental boron, TiB2 or as any appropriate master alloy containing boron. The boron may be added in amounts in the range from 0.01% to 18.4%, by weight. More preferably, the boron is added to the titanium alloy in amounts ranging from 0.01% to 2%, by weight, depending on titanium alloy composition.
Hot metalworking operations such as forging, rolling, and extrusion can be used to accomplish alignment of TiB precipitates along the direction of metal flow.
The present method can be practiced by the gas atomization powder metallurgy process flowchart shown in
The powder compact is then subjected to a metalworking operation such as forging, rolling, or extrusion. Hot working parameters commonly practiced for producing titanium alloy articles were found to produce the desired alignment of TiB precipitates along the direction of metal flow. As an illustrative example, the hot working parameters are as follows:
Micrographs at different locations of a Ti-6Al-4V-1B article made via forging of a powder compact of 16″ height×3.5″ diameter into a disk of 3″ height×8″ diameter in the temperature range 1750-2200° F. and a ram speed of 40 inch/min are shown in
Thermal and electrical properties of several TiB incorporated titanium alloy articles (chemical compositions given in Table 1) were evaluated. Identical testing was performed on titanium alloys without TiB precipitates for comparison. Thermal conductivity testing was performed in accordance with the standard test method ASTM E1461 and electrical resistivity was determined per the standard method ASTM B84.
Thermal conductivity of Ti-64-1B (labeled as nano Ti-64) forging and extrusion articles is compared with that of Ti-64 article in
Thermal conductivity data of Ti-6242-1B forging article is compared with that of the baseline Ti-6242 article in
Electrical resistivity of Ti-64-1B (labeled as nano Ti-64) forging article is compared with that of Ti-64 article in
In addition to the improvements in thermal and electrical properties, TiB incorporated titanium alloys offer several benefits in mechanical properties without debits in ductility and fatigue. For example, room temperature tensile properties of boron-modified titanium alloy articles (referred to as nano version) are compared with those of baseline titanium alloys in Table 2. In nano titanium alloys, the tensile yield strength and ultimate strength were higher by 25%, modulus of elasticity is higher by 20%, while maintaining tensile elongations equivalent to their baseline titanium alloys.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.